The present invention relates to guided missiles.
During flight at all but the lowest angles of incidence the air flow separates over the leeward side of slender bodies to form vortices. This vortex flow can be symmetric, but is more usually asymmetric with the result that a side force (and yawing moment) is generated in addition to the normal force on the body in the pitch plane. This `out-of-plane` force is undesirable in guided missiles in that it complicates the control of maneuvering.
Various means have been sought to reduce, or eliminate, this out-of-plane force but with limited success. One class of solution is to modify the nose region, for example by providing strakes to anchor the vortices and their development. A pair of such strakes has been successful, but only in respect of one roll orientation of the body, i.e. the strakes have to be substantially symmetrical about the pitch plane. The use of a plurality of bodies or strakes, rings or transition bands, all around the nose area, was discussed in the 1972 AIAA Paper 72/968 "Occurrence and inhibition of large yawing moments during high incidence flight of slender missile configurations" by William H Clark et al. They too are not entirely successful and anyway have undesirable drag penalties.
Another class of solution is to `average-out` the asymmetries, e.g. by a continuously-rotating nose section; this approach, which is described in NEAR Inc's Technical Report 212 of December 1979, "Active Control of Asymmetric Vortex Effects" by John E Fidler, has the merit of applying at all roll orientations but has been found not fundamentally to reduce the magnitude of the out-of-plane forces.